The invention is connected with optical communication systems in which digital signals are to be transmitted at a high bit rate, e.g., 40 Gb/s. At such a high bit rate, electrical system components must be optimized at high cost. Above an upper cutoff frequency, some electrical system components, such as devices for clock recovery, are no longer usable.
A device for clock recovery is, for example, an optical phase-locked loop as is known, for example, from Saruwatari, M. et al, "Optical Signal Processing for Future Communications Networks", NTT Review, Vol. 4, No. 6, November 1992, pages 83 to 91.
FIG. 2 of that article shows an optical phase-locked loop (PLL) which includes a laser-diode amplifier (LDA) used as a phase detector, an oscillator (VCO), and an optical clock generator. An experimental result for a 10 Gb/s signal is shown, and the circuit is expected to operate at up to 100 Gb/s.
The laser-diode amplifier mixes the signal light of wavelength .lambda..sub.1 with light of wavelength .lambda..sub.2 which is emitted by the optical clock generator. The laser-diode amplifier is thus an optical mixer for a communications system. The upper cutoff frequency of the optical phase-locked loop is determined primarily by the cutoff frequency of the laser-diode amplifier.
This optical phase-locked loop is disadvantageous in that the clock signal used for mixing, which is generated by the optical clock generator, has a frequency slightly different from that of the signal light, namely 10.0001 GHz.
It is an object of the invention to provide another optical phase-locked loop. This object is attained by an optical phase-locked loop comprising an optical clock generator, which has an optical mixer and a control section which converts light emerging from the optical mixer and composed of signal light, pump light, and a mixing product into an electric control signal corresponding to an intensity difference between the signal light and the mixing product, and that the optical clock generator, which is controlled by the control signal, is connected to the optical mixer, so that the latter receives a portion of the optical clock as the pump light.
It is another object of the invention to provide an optical mixer for a communications system, comprising a length of optical waveguide as a mixing element and a coupler for coupling signal light and pump light into a first end of the length of optical waveguide, from whose second end emerges the signal light, the pump light, and a mixing product thereof, that the length of optical ;waveguide is polarization-maintaining, and that the pump light, which is plane-polarized, is coupled into the length of optical waveguide so that its plane of polarization makes an angle of approximately 45.degree. with the principal axes of the length of optical waveguide. It is also directed to the use of the optical mixer as an optical phase-locked loop.
Further advantageous aspects of the invention are claimed in the subclaims.
The invention uses the four-wave mixing principle. This principle is known, for example, from Inoue, Kyo, "Four-Wave Mixing in an Optical Fiber in the Zero-Dispersion Wavelength Region", Journal of Lightwave Technology, Vol. 10, NO. 11, November 1992, pages 1553 to 1561.
There, an experimental setup is shown with which four-wave mixing (FWM) in a fiber can be examined. Four-wave mixing is an unwanted phenomenon which causes system degradation, particularly in multichannel transmissions.
The experimental setup includes a signal laser, a pump laser, a coupler, and a fiber to be tested. The light resulting from the mixing in the fiber is analyzed in a spectrum analyzer. To ensure that the amount of power of the signal and pump lights in the fiber is large enough, the signal and pump lights are amplified by means of optical amplifiers before entering the coupler. The resulting mixing is referred to as "degenerate four-wave mixing", and the mixing product as "fourth wave". To maximize the optical power of the mixing product, the polarization states of the signal and pump lights are varied by means of a polarization controller.